Adsorption is a process by which a gas, liquid, or dissolved material is assimilated onto the surface of a solid or liquid material and defined in terms of adsorptive surface area per unit mass. In contrast, an absorption process entails incorporation of materials into the pores or interstitial spaces, as opposed to only the surface, of an absorbent material. An adsorbing material/adsorbent or an absorbing material/absorbent is called sorbent. A material being sorbed (either adsorbed or absorbed) is called the sorbate (either adsorbate or absorbate).
A number of different factors and mechanisms influence the adsorption process. For example, polar molecules are often more easily adsorbed. Similarly, molecules with small kinetic diameters can be preferentially adsorbed relative to molecules with larger kinetic diameters. Additionally, the condensation characteristics of the sorbate can also affect the adsorption process. Furthermore, the quadrupole moment of a molecule may make it more easily adsorbed than another molecule. Accordingly, adsorption systems can manipulate these factors and mechanisms to separate components of complex mixtures and/or to effect selective vapor condensation.
A simple, traditional adsorption system has two separate vessels filled with sorbent material. The sorbents are often complex chemical structures having powerful attractive forces and are capable of higher degrees of selectivity and molecular discrimination than membrane filters. A mixture is passed over the sorbent material of one of the vessels causing a component of the mixture to be removed from the feed stream. Once the sorbent in the first vessel is no longer able to adsorb any more material, the feed stream is switched to the second sorbent containing vessel. While the second vessel is adsorbing, the first vessel is being purged (i.e. desorbed) of the adsorbed material. Thereafter, the first vessel is substituted for the second vessel while the second vessel is purged. This process, known as swing adsorption, is repeated as needed.
The material handling capacity of such adsorption systems depends on a number of variables, including vessel size (i.e. sorbent mass), cycle time and operating pressure, as well as sorbent/adsorbate affinity. For example, increasing vessel size, and hence the volume and mass of sorbent, increases adsorption capacity. Similarly, decreasing the cycle time provides a concomitant increase of available adsorption sites per unit time. Increasing the operating pressure of the system also increases adsorption capacity per unit volume.
Increasing the cycle rate of a sorptive system requires decreasing the relative size of the adsorbent particle to increase the diffusion rate into and out of the adsorbent particle. Decreased particle size undesirably increases the rate and likelihood of co-adsorption of unwanted molecular moieties. Ultra-rapid cycle adsorption systems, therefore, require methods of preventing co-adsorption, or means of periodically regenerating or cleansing the adsorbent.
Liberation of the sorbed material from the sorbent (i.e. desorption) can occur via a number of different mechanisms. Conventional adsorption systems employ either pressure reduction or temperature increase for removal of the adsorbate. Systems swinging between adsorption and pressure differential desorption are known as pressure swing adsorption (PSA) systems. Alternatively, adsorption systems switching between adsorption and temperature differential desorption are known as temperature swing adsorption (TSA) systems. Other desorption mechanisms exist, including electrical energy desorption (for dielectric and/or conductive sorbents) and microwave irradiation of sorbent/adsorbate complexes.
Regardless of the adsorption/desorption process employed, these systems require that an energy balance be maintained in the system. That is, energy that is dissipated during adsorption (as heat) must be reintroduced into the system during desorption. The most efficient adsorption systems, in terms of energy, are those containing the least amount of superfluous mass because heating and cooling a large vessel, a large volume of sorbent, and associated binder materials during the repeating cycles is a very wasteful process. As a result, the current trend is toward lower mass, rapid cycle systems despite the fact that such measures have traditionally been associated with reducing volumetric efficiency.
Recent advances in the field of micro electromechanical systems (MEMS) research have led to proposals for incorporating micro-channel adsorption and reaction devices that provide for very short cycles with increased heat transfer capacities into traditional PSA and TSA systems. Such devices alternate the flow and pressure of complex compounds into and from sorbent filled micro-channels (thus increasing surface area with minimal effect on system size). For example, corrugated sheets have been impregnated or covered with thin layers of such sorbent materials. Additionally, such systems offer the possibility of exceedingly short cycle times on the order of tenths of seconds. Accordingly, it is envisioned that such devices would be particularly well suited for use in small devices, such as oxygen enrichment systems for hospital patients. However, these devices still require mechanical valving and compressors that may lead to mechanical failure and unwanted bulk.
Another method of separating and/or purifying includes membrane technology. Membranes function like filters in that they allow certain substances to pass through them while preventing others from passing through their pores and remain in the feed stream. Membranes are very basic in their operation and require no special valving, switching, or purging cycles. Membranes may function in a continuous manner. However, membranes provide no effective means for removing the substances trapped in their pores, they have a high power requirement, and they are limited in their selectivity. Also, membranes require high pressure differentials to effect diffusion and large surfaces because of low permeance per unit area. Membranes can also incorporate sorbent materials to improve selectivity. These types of membranes can be operated in a pressure swing manner, where the desorption portion of the cycle serves to clean the membrane pores of adsorbed material.
The need to decrease the size and mechanical complexity of adsorptive fluid separation and thermal lift devices is driven by economic concerns and by the need to integrate these devices into increasingly efficient and durable micro systems. For example:                Manned space platforms require environmental remediation apparatus of extremely low mass and high reliability. Current carbon dioxide removal systems require large quantities of sorbent material.        Hybrid-, electric-, and fuel cell-powered land vehicles will require non-mechanical solutions to meet cabin heating and cooling needs and will likely be powered by a 42-volt electrical system. Current thermal comfort systems rely on waste heat and mechanical energy that will not be available in future vehicle designs.        The use of fuel cells as power sources for automotive, generation of electricity, and portable electronics necessitates the development of small, lightweight fuel reformers, oxygen concentrators, and fuel purification devices. This need is currently supplied by membrane and pressure swing adsorption machinery that is both bulky and energy consumptive.        The use of oxygen enriched air for combustion processes in transportation, metal refining, and chemical processes, and in pollution abatement, agriculture, and aquaculture is being developed. The economic viability of these proposed systems enhancements requires that oxygen-nitrogen separation equipment be affordable, energy efficient, and, in the case of propulsion applications, able to meet mass-volume criteria.        The need to create temperature conditioned spaces for human habitation, food storage, and for sensitive equipment is increasing along with the expectations and living standards of the human population. This is applying tremendous strains on energy production capacity and material availability. Present mechanical thermal lift devices are material intensive, complicated, and use working fluids that contribute to the “greenhouse effect.” Mechanical systems also have high starting currents requiring oversized electrical supply systems.        Industrial operations requiring separation technology to provide feed stocks utilize large scale pressure and temperature swing adsorption, distillation, and compressor driven membrane equipment that, because of “economy of scale” considerations, obviates the adoption of more efficient point-of-use manufacturing practices.        
Some improvements have been made toward decreasing the cycle times of TSA and PSA systems. In one instance, a plurality of sorbent containing pressure vessels is held central to a continuously rotating valve assembly. This results in lower fluid residence time in the reactor vessel and higher throughput per unit volume of sorbent. In another instance micro reaction chambers are created by etching or otherwise forming linear channels on a substrate. The substrate is formed of or contains sorbent material. This allows for heat exchange between reaction channels and for short reaction times. Both of these configurations are vast improvements over existing pressure and temperature swing adsorption systems that rely on vessel size and operating pressure to increase capacity.
There is, however, a continuing and pressing need for methods and devices that are capable of selectively separating and/or purifying mixtures, particularly those that reduce cycle times even further, minimize or eliminate moving parts, valving, switching, and purging, and that may function continuously, and are lightweight and portable. There is also a need for devices and systems that are less expensive to build, less complicated to maintain, scalable, and energy efficient, especially for mobile and small-sized applications. Furthermore, there is a need for technology that enhances the sorption and desorption of these methods, systems, and devices to optimize their performance. In addition, there is a need to maintain the dryness of the adsorbent to optimize capacity. Methods are also needed that can recoup the considerable energy produced during the desorption phase of the cycle. The invention is directed to these, as well as other, important ends.